Cr-induced cellular injury and necrosis in Glycine max L.: Biochemical mechanism of oxidative damage in chloroplast.

Chromium-induced toxicity and mechanisms of cell death involved in plants are yet to be fully elucidated. To understand the events of these processes, the stress response of the soybean plant using trivalent and hexavalent chromium compounds, namely, basic chromium sulphate (BCS) and potassium dichromate (K2Cr2O7) was investigated. The leaf surface morphology for stomatal aperture, wax deposition and presence of trichomes for chromium accumulation was examined by SEM-EDAX and light microscopy. The leaf mesophyll cell integrity was identified by trypan blue staining; chlorophyll autofluorescence, ROS generation and mitochondrial function were studied by fluorescence microscopy using different dyes. Isolated chloroplasts were analysed for micronutrients and total chromium content by AAS. Elevated Cr level and decreased Fe, Cu and Zn content in chloroplast revealed the active transportation of highly soluble Cr6+ species resulting in poor absorption of micronutrients. Cr accumulation as Cr(V) in chloroplast was noticed at g = 1.98 of electron paramagnetic resonance signal. Plants grown in Cr(VI) amended soil showed chemical modification of biological macromolecules in the chloroplast as observed from fourier transform infra-red (FTIR) spectra; the chloroplast DNA damage was confirmed by DAPI staining. Cr(VI)-treated plants showed significant reduction in the levels of various biochemical parameters. The results altogether clearly indicate that Cr(VI)-induced reactive oxygen species (ROS) production leads to oxidative stress-associated changes in the organelles, particularly in chloroplast, resulting in cell death.

Inducible chromate reductase exhibiting extracellular activity in Bacillus methylotrophicus for chromium bioremediation.

Chromium toxicity is one of the major causes of environmental pollution due to its heavy discharge in industrial wastewaters. Chromate reduction is a viable method to detoxify hexavalent chromium to nontoxic trivalent species mediated by enzymes and metabolites. A new Bacillus methylotrophicus strain was isolated from tannery sludge and was an efficient candidate for chromate reduction. An initial chromate reductase activity of 212.84 U/mg protein was obtained at 48 h in a low-cost defined medium formulation with 0.25 mM chromate. The extracellular enzyme was inducible at 12h substrate addition with 312.99 U/mg at 48 h. Reduced glutathione was required for enhanced specific activity of 356.48 U/mg. Enzyme activity was optimum at pH 7.0 and at 30 °C, and was stable in the presence of EDTA, DTT and metal ions. The enzyme exhibited a Vmax of 59.89 µM/min/mg protein and a Km of 86.5 µM, suggesting feasibility of the reaction with K2Cr2O7 as substrate. Application of the crude reductase in tannery effluent resulted in 91.3% chromate reduction at 48 h. An enzyme-mediated chromate reduction process has therefore been developed for bioremediation of toxic chromium sp. in industrial effluents.